Phase contrast imaging apparatus

ABSTRACT

An x-ray imaging system comprising an x-ray source, an x-ray detector comprising a plurality of detector strips arranged in a first direction of the x-ray detector, each detector strip further comprising a plurality of detector pixels arranged in a second direction of the x-ray detector; a phase grating; a plurality of analyzer gratings comprising grating slits; a phase grating, and a plurality of analyzer gratings comprising grating slits, wherein the x-ray source and the x-ray detector are adapted to perform a scanning movement in relation to an object in the first direction, in order to scan the object, wherein the analyzer gratings are arranged between the x-ray source and the x-ray detector, wherein each of the plurality of analyzer gratings ( 162 ) is arranged in association with a respective detector strip with the grating slits arranged in the second direction and wherein the grating slits of the analyzer gratings of the detector strips are displaced relative to each other in the second direction.

TECHNICAL FIELD

The present invention relates generally to an improved x-ray imagingapparatus in the field of mammography, tomosynthesis, radiography,wherein phase contrast imaging capabilities have been implemented.

BACKGROUND ART

In prior art it has been proposed to use phase contrast in x-ray imagingto increase signal-to-noise ratio (SNR) in e.g. mammographicapplications. Medical x-ray imaging is often limited by small contrastdifferences and high noise caused by tight dose restraints. This isparticularly true for mammography where low contrast tumors constitute amajor detection target, and a large number of tumors are missed ormisdiagnosed due to difficulties in detection. The use of phase contrastimaging in medical applications have shown promising in order toincrease SNR, since the phase shift in soft tissue is in many casessubstantially larger than the absorption.

International patent application WO 2008/006470 A1 describes the use ofinterferometers for x-rays wherein x-ray images can be acquired from ascanned object. The set up herein comprises means for evaluatingintensities on a pixel 105 p basis in order to identify characteristicof the object 108 by characterizing each pixel 105 p as being forinstance phase contrast or absorption contrast dominated. In oneapplication concerning the investigation of luggage on a moving conveyorbelt, a set-up comprising an array of line detector 105 s and a numberof sub-gratings are arranged between the object 108 and the linedetector 105 s wherein each of the sub-gratings are shifted in theirposition perpendicular to the grating lines. In this manner, luggage tobe investigated is moved along a direction perpendicular to the gratinglines during a scan, wherein one scan movement is required to acquirephase contrast and absorption contrast data.

There are a number of disadvantages with the art presented above. Firstof all, the solution requires the manufacturing of physically longsub-gratings G1 _(n) and G2 _(n), which is consumes resources in termsof cost and time.

Another drawback is that the proposed set up if directly implemented ina mammography application is likely to induce errors in the phasedetection due to the direction of the scan vs the direction of phasecontrast detection. When a stationary object 108 such as a breast isanalyzed, the set up needs to be moved in a scan direction in relationto the object 108 to create an x-ray image of the object, not the otherway around as in WO 2008/006470 A1. An example of a scanning x-raymammographic imaging system with absorption contrast functionality canbe seen in document X, by the applicant. In WO 2008/006470 A1, the scandirection is set to be perpendicular to the grating lines of thesub-gratings, and hence perpendicular to the interference fringes 163 tobe detected. It is well known that any system, including scanningsystems, generally introduce more disturbances in the scan directionsince this is the direction of change. Another source of disturbance isthe gravity itself on a moving scan arm 103, since a gravity componentof the scan arm 103 will induce a torque on a detector 105/analyzergrating 162 relative to the beam 122 splitter. Hence, a small shiftbetween the analyzer grating 162 and the beam 122 splitter due togravitation will further induce errors in the phase detection. Inconclusion, in order to reduce these disturbances a set up as in theprior art needs have very high requirements on scan precision, whichwill make the manufacturing of such products more costly as well astime-consuming.

Yet another drawback of the prior art is that the full potential ofusing phase contrast, especially sought for in mammographicapplications, is not utilized. One of the main advantages of using phasecontrast imaging is the reduced noise at high spatial frequencies, i.e.an improved ability to detect small features. In a scanning system,spatial resolution is generally lower in the scan direction sincecontinuous read out is most often implemented.

SUMMARY OF INVENTION

An object of the present invention is to alleviate some of thedisadvantages of the prior art and to provide an improved device forx-ray imaging. According to one embodiment, the x-ray imaging systemcomprises an x-ray source, an x-ray detector comprising a plurality ofdetector strips arranged in a first direction of the x-ray detector,each detector strip further comprising a plurality of detector pixelsarranged in a second direction of the x-ray detector; a phase grating; aplurality of analyzer gratings comprising grating slits; a phasegrating, and a plurality of analyzer gratings comprising grating slits,wherein the x-ray source and the x-ray detector are adapted to perform ascanning movement in relation to an object in the first direction, inorder to scan the object, wherein the analyzer gratings are arrangedbetween the x-ray source and the x-ray detector, wherein each of theplurality of analyzer gratings (162) is arranged in association with arespective detector strip with the grating slits arranged in the seconddirection and wherein the grating slits of the analyzer gratings of thedetector strips are displaced relative to each other in the seconddirection.

According to another embodiment, the x-ray imaging system (101)comprises an x-ray source (104), an x-ray detector (105) (1 comprising aplurality of detector strips (105)a) arranged in a first direction ofthe x-ray detector (105), each detector strip (105)a further comprisinga plurality of detector (105) pixels (105 p) arranged in a seconddirection of the x-ray detector (105); a phase grating (161); and aplurality of analyzer gratings (162) comprising grating slits; whereinthe (1) x-ray source (104) and the x-ray detector (105) are adapted toperform a scanning movement in relation to an object (108) in the firstdirection, in order to scan the object; wherein the analyzer gratings(162) are arranged between the x-ray source (104) and the x-ray detector(105), wherein a plurality of analyzer gratings (162) is arranged inassociation with a respective detector strip (105 a) with the gratingslits arranged in the second direction and wherein the grating slits ofthe analyzer gratings (162) of the detector strips (105 a) are displacedrelative to each other in the second direction.

According to another embodiment, the displacement of grating slits ofthe analyzer gratings (162) along a plurality of detector strips (105 a)samples an entire fringe period (163 d) of interference fringes (163)generated by a phase grating and displaced by a phase gradient in theobject (108) when the object (108) is scanned.

According to another embodiment, the grating slits of analyzer gratingsof two consecutive detector strips with analyzer gratings in a firstdirection are displaced relative to each other in the second directionin a systematic manner, wherein the systematic manner comprises adefined displacement distance.

According to another embodiment, the displacement distance (d) is afraction of the fringe period p_(f), such that

${\frac{p_{f}}{N} < d < p_{f}},$

where N is the number of detector strips such that the entire fringeperiod 163 d is covered.

According to another embodiment, the displacement distance (d) isbetween,

${\frac{p_{f}}{3} < d < p_{f}},$

preferably

$\frac{p_{f}}{3}.$

According to another embodiment, the grating slits of analyzer gratingsof two consecutive detector strips are displaced relative to each otherin the second direction in an arbitrary manner, wherein the arbitrarymanner comprises a random displacement.

According to another embodiment, the grating slits of the randomlydisplaced analyzer gratings (162), when summarized, are uniformlydistributed over an entire fringe period.

According to another embodiment, two consecutive detector strips withanalyzer gratings in a first direction are two adjacent detector strips.

According to another embodiment, two consecutive detector strips (105 a)with analyzer gratings (162) in a first direction are randomly orarbitrarily displaced among the detector strips (105 a) in a firstdirection.

According to another embodiment, the system is adapted to be calibratedsuch that the exact position of the analyzer gratings is established.

According to another embodiment, analyzer gratings are arranged on alldetector strips.

According to another embodiment, system further comprises apre-collimator and a post-collimator, wherein the pre-collimator isarranged between the analyzer grating and the phase grating and thepost-collimator is arranged between the analyzer grating and the x-raydetector.

According to another embodiment, the system further comprises a sourcegrating arranged between the x-ray source and the phase grating.

According to another embodiment, the detector (105) is adapted to countphotons impinging on the detector strips (105 a) and generate a signalcorresponding to the energy of impinging photons, and wherein a controlunit (121) is adapted to receive said signals and assign a weight to thephase-contrast image effect in relation to the efficiency at each energyand/or wherein the control unit 121 is adapted to assign a weight to thephase-contrast image effect in relation to the efficiency at eachenergy.

According to another embodiment, the control unit (121) is adapted toassign a higher weight of photons within a first energy interval to thephase-contrast image effect wherein phase contrast is more optimal,and/or wherein the control unit is adapted to assign a higher weight ofphotons within a second energy interval to the absorption contrasteffect, wherein absorption contrast is more optimal.

According to another embodiment, the first and second energy intervalsare defined by a first, lower threshold value of the photon energy, anda second, higher threshold value of the photon energy, wherein eachdetector pixel of each detector strip is connected to a comparator andcounter comprising at least two threshold values for comparing thesignal with said threshold values and counting said photons within thefirst and second energy intervals.

According to another embodiment, that the detector is adapted to counteach photon impinging on the detector strips and generate a signalcorresponding to the energy of each impinging photon, and whereinphotons within an energy interval comprising a lower energy threshold, ahigher energy threshold, wherein the interval comprises an optimalenergy for phase contrast are readout to enhance the phase contrastimage effect

According to another embodiment, that the energy distribution depends onthe set voltage of the x-ray source or on the breast thickness, whereinthe control unit is adapted to receive signals comprising informationregarding the set voltage and/or receive signals comprising informationregarding the breast thickness, for instance from an automated exposurecontrol which optimizes the voltage based on the thickness of theobject, and adapts the lower energy threshold and the higher energythreshold based on this information.

According to another embodiment, first energy interval contains higherphoton energies than the second energy interval.

According to another embodiment, at least one analyzer grating isarranged in a first crosswise direction over the entire detector.

According to another embodiment, the system further comprises at leastone movable compression paddle, wherein the compression paddle isadapted to move an object, such as a breast, further away from theanalyzer grating to increase the phase contrast image effect.

According to another embodiment, the at least one compression paddle isadapted to move the object within a range between the analyzer gratingand the pre-collimator or the phase grating.

According to another embodiment, the system further comprises a controlunit adapted to move the compression paddle into a position wherein theratio of the phase contrast and absorption contrast is optimized.

According to another embodiment, the at least one compression paddle isadapted to be arranged below an object.

According to another embodiment, a scan arm is provided, wherein thex-ray source is arranged in a first position of the scan arm and thex-ray detector is arranged in a second position of the scan arm.

According to another embodiment, the phase grating is arranged on thescan arm in order to follow the scan arm during the scanning movement inrelation to an object in the first direction.

According to another embodiment, the phase grating is arranged to bestationary wherein the scan arm during the scanning movement is moved inrelation to an object and the phase grating in the first direction.

According to another embodiment, the analyzer grating is arranged oneach of a plurality of detector strips.

According to another embodiment, the analyzer grating is connected toeach of a plurality of detector strips by a snap-fit like connection.

According to another embodiment, the analyzer gratings (162) arearranged directed towards the x-ray source (104), wherein the tiltingdirection of the analyzer gratings (162) are essentially equal to thetilting angle(s) of the plurality of detector strips (105 a) in relationto the x-ray source (104)

According to another embodiment, each analyzer grating 162 comprisesseveral smaller units, adapted to be connected to each other during themanufacturing of the analyzer gratings 162.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 shows a perspective view of an x-ray imaging system

FIG. 2 a shows a schematic view of the x-ray imaging system set up in ax-z-plane, corresponding to the phase contrast plane

FIG. 2 b shows a schematic view of the x-ray imaging system set up asseen in FIG. 2 a in a y-z plane, corresponding to the absorptioncontrast plane

FIG. 3 a shows a portion of the detector and a systematic displacementof the analyzer grating of adjacent detector strips

FIG. 3 b shows portion of the detector and a systematic displacement ofthe analyzer grating of non adjacent detector strips

FIG. 3 c shows a portion of the detector and a random displacement ofthe analyzer gratings 162 of adjacent detector strips

FIG. 3 d shows a portion of the detector and a systematic displacementof the analyzer grating arranged in a perpendicular direction in across-wise manner

FIG. 4 a shows the detector and a set up for energy weighting

FIG. 4 b shows energy distribution of photons of an x-ray source

FIG. 5 shows a compression paddle in two alternative positions.

DESCRIPTION OF EMBODIMENTS

In the following, a detailed description of the invention will be given.

FIG. 1 illustrates an x-ray imaging system 101 according to oneembodiment, based on a photon-counting detector 105 that scans the imagefield in one direction. The system according to this embodiment is basedon the existing scanning systems for x-ray imaging developed by theapplicant, whereby the system have the same external features as are forinstance known from document U.S. Pat. No. 7,496,176. The system thuscomprises an x-ray source 104 arranged in a housing, patient support andpre-collimator 106 a housing and compression paddle 107 a, 107 b. Acollimator is arranged in a collimator support, and the patient supportcomprises a detector 105 comprising a plurality of detector strips 105a. The x-ray source 104 and the detector 105 are arranged essentially inrespective ends of a scan arm 103, hence arranged to be displacedradially with the x-ray source 104 in the centre. An image is acquiredby scanning the detector 105 across an image field and applyingabsorption contrast principles. Whenever the detector 105 has scanned apredefined distance, the number of photon counts collected is read-outand the counter is reset. However any other type of x-ray system mayimplement the phase contrast imaging capability described herein,preferably systems with a scan movement to cover and generate an imageof an object, by irradiating the object with x-ray beams.

FIG. 2 a, disclosing one embodiment of the invention, shows a schematicview of a cross-section of the x-ray imaging system 101 enabling phasecontrast imaging, set up in a x-z plane as defined by the coordinatesystem seen in the figure. The system according to this embodimentcomprises a Talbot interferometer set up, and is thus based on so-calledTalbot interferometry, also known as grating interferometry,grating-based phase contrast imaging, or differential phase contrastimaging, wherein phase shift is inferred by intensity differences,generated by placing a number of gratings in the beam 122 path. The scandirection along a radial path, with the x-ray source 104 in the centeras seen in FIG. 1, is defined to be in the y-z plane, denoted by thearrow in FIG. 2 b. In FIG. 2 a the x-ray source 104 is arranged at theuppermost position of the system radiating the detector 105 arranged atthe lowermost position in the figure. The x-ray source 104 emits anx-ray radiation beam 122. In one embodiment, a source grating 160 isarranged slightly displaced from the x-ray source 104 in the directionof the x-ray radiation field towards the detector 105. The sourcegrating 160 lines extends in a y-direction. The purpose of the sourcegrating 160 is to generate an array of small x-ray sources 104, whichimproves photon economy substantially compared to a single small x-raysource 104 without reducing coherence. Further down in the direction ofthe x-ray radiation field, a phase grating 161, sometimes denoted beam122 splitter, is arranged with the purpose to introduce an effect knownas Talbot self images, which are interference fringes 163 that appear atperiodic distances from the grating and parallel with the gratingstrips, also known as grating lines. A pre-collimator 106 a may bearranged essentially adjacent the phase grating 161, as seen in thisembodiment to further enhance dose efficiency by illuminating only theparts of the object that can be seen with the detector. To facilitatethe understanding of the workings of the phase contrast effect, anexemplary triangularly shaped object 108 is arranged between the phasegrating 161 and the detector 105 according to this embodiment, howeversaid object 108 but may also be arranged between the x-ray source 104and the phase grating 161 and achieve similar effects. The object 108corresponds to e.g. a breast in a mammography application. Slightlyabove the detector 105 as seen in a direction towards the phase grating161, an analyzer grating 162 is arranged. The analyzer grating lines 162a extends in first direction corresponding to the scan direction y,whereas a plurality of grating slits, i.e. openings 162 b, of theanalyzer grating 162 extends in a second direction x, perpendicular tothe scan direction, whereas the analyzer grating 162 extends a longerdistance in the first direction than in the second direction. The pitch162 d of the analyzer grating 162 is referred to as the distance betweenthe center of two adjacent closures 162 c of the grating as can be seenin FIG. 2 a, or in other words, as the total length of the width of oneopening and one closure 162 c in the grating. Between the analyzergrating 162 and the detector 105 a second post-collimator 106 b may bearranged to further reduce photon scattering and thereby improve doseefficiency. Thus, as seen in the figure, the phase grating 161 isilluminated by the x-ray source 104 which is covered by a source grating160 and induces interference fringes 163. The fringes 163 are displacedby the phase gradient, i.e. the derivative of the phase shift, in theobject, and the fringe period 163 d remains constant. The fine-pitch 162d analyzer grating 162 arranged in association with the detector 105 canbe used to derive the fringe displacement and hence the phase shift.

For a spherical beam 122 induced by a point source, the so-called Talbotdistances are:

${d_{n} = \frac{D_{n}L}{L - D_{n}}},{where}$${D_{n} = \frac{{np}_{1}^{2}}{2\eta^{2}\lambda}},{n = 1},3,5,\ldots$

Here, L is the source-to-grating distance, n is the Talbot order, p₁ isthe pitch 162 d of the phase grating 161, λ is the x-ray wavelength, andη is the parameter that depends on the phase grating 161 type.

Assuming a π phase-shifting phase grating 161 according to oneembodiment, which implies η=2. For a π/2-shifting phase grating 161,η=1.

The period of the interference fringes 163 is

${p_{f} = \frac{P_{f}L}{L - D_{n}}},{where}$$P_{f} = \frac{p_{1}}{\eta}$

Again, P_(f)=p_(f)(L→∞) is the fringe period 163 d for a plane incidentwave. If the source is covered with a source grating 160 with openings162 b that are s wide and with a pitch 162 d of

$p_{0} = {p_{f}\frac{L}{D_{n}}}$

The Talbot images generated from the different source slits coincide andgenerate a higher flux, which is of relevance to keep down the exposuretime in phase-contrast imaging.

When a phase shifting object 108 is introduced in the beam 122, it isrefracted an angle α=Φ′λ/2π, where Φ′ is the differential phase shift ofthe object. For small α, the refraction causes a fringe displacement

${{\Delta \; p_{f}} \approx {\Lambda \; d_{n} \times \alpha}} = {\Lambda \; d_{n}\frac{\lambda}{2\pi}\Phi^{\prime}}$

at a distance Λd_(n) from the object, where Λ ranges from 0 for anobject 108 placed in contact with the detector 105 to 1 for an object108 at or upstream of the phase grating 161. The fringes 163 areperiodic as a function of x, i.e. in the x direction in the set upaccording to the embodiment. Thus, a phase gradient in the object 108causes a phase shift of the fringes 163, which can be measured to obtainΦ′ by the intensity variations sensed behind the analyzer gratings 162.The phase shift Φ may be obtained by integration of Φ′. The placing ofthe analyzer gratings 162 before the detector 105 is not theoreticallynecessary in order for a detector 105 to sense the fringe displacement;however it reduces the resolution requirement of the detector 105.Detectors 105 with enough resolution to detect Δp_(f) may in fact bedifficult and expensive to manufacture. One method used in the pastcomprising the analyzer grating 162 is the step-wise movement of theanalyzer grating 162 in the x-direction until the entire fringe period163 d is covered by the openings 162 b of the analyzer grating 162,preferably in at least M=3 measurements or steps. Such methods arenormally referred to as phase stepping methods.

FIG. 2 b shows a cross-section of the schematic view of the x-rayimaging system 101 set up as seen in FIG. 2 a in a y-z plane. From thisdirection, it is made clear that the set up comprises a multi-slitgeometry, according to well-known principles developed by the applicant,wherein the detector 105 comprises a plurality of Si strip detector 105s aligned with each of the plurality of slits of the pre-collimator 106a and post-collimator 106 b. According to one embodiment, 21 stripdetectors 105 are preferably used in a detector 105. Above and inassociation to each of a plurality of the detector strips 105 a,analyzer gratings 162 have been arranged. As stated earlier, a scanningmovement takes place in the y-z plane, essentially in the y-direction,as is also shown by the arrows in the figure. This direction is thusadapted to measure absorption contrast of an object, which will befurther described below. According to one embodiment, the analyzergratings are directed towards the x-ray source to minimize losses causedby the high aspect ratio of the analyzer gratings, i.e. in order toincrease the dose efficiency and reduce scatter, the openings of theanalyzer gratings are made more aligned with the direction of the x-raybeams. According to one embodiment, such direction may require tiltingof the analyzer gratings such that they essentially have their openingperpendicular to the incident x-ray beams, similar to the direction ofthe surface of each detector strip 105 a.

According to one embodiment, each analyzer grating 162 comprises severalsmaller units, adapted to be connected to each other during themanufacturing of the analyzer gratings 162.

FIG. 3 a shows a portion of the detector 105 comprising four detectorstrips 105 a, the figure essentially viewed in a direction of incidentx-ray beams 122 towards the detector 105, essentially in a negativez-direction according to the coordinate system of FIG. 2 a and FIG. 2 b.Four analyzer gratings 162 are arranged in association to these detectorstrips 105 a, i.e. they are arranged in manner along the detector strips105 a in a second direction x to alternately cover and not cover thedetector strips 105 a with the openings 162 b and closures 162 c of theanalyzer gratings 162. Further, each detector strips 105 a are built upby a plurality of detector 105 pixels 105 p arranged side-by-side in anx-direction as seen in the figure. To facilitate the illustration of theset up, the pixels have been made essentially rectangular. However, theymay have any other type of shape. In one embodiment the analyzergratings 162 are arranged directly on the detector strips 105 a, forinstance by a snap-fit connection between the analyzer gratings and thedetector strips, but in another embodiment there is a slight distancebetween the detector strips 105 a and the analyzer gratings 162. In yetanother embodiment, the analyzer gratings 162 are arranged directly onthe post-collimator 106 b which in turn is arranged directly on thedetector 105. According to the embodiment of FIG. 3 a, the analyzergratings 162 are arranged slightly displaced relative to each other insystematic manner in an x-direction, perpendicular to the scandirection, wherein the displacement is essentially equal for twoconsecutive detector strips 105 a with analyzer gratings 162. Thedisplacement distance d, i.e. the displacement of grating slits of theanalyzer gratings 162 along a plurality of detector strips 105 a isdefined such that the plurality of detector strips 105 a with analyzergratings 162 samples an entire fringe period 163 d of interferencefringes 163 generated by a phase grating and displaced by a phasegradient in the object 108 when the object 108 is scanned during thescanning movement in the y-direction. In this manner, no additional scanis required in the x-direction according to e.g. a step-scan approach,since the fringe period 163 d in the x-direction is scanned along withthe scan movement in the y-z-plane.

The displacement d is a fraction of the fringe period 163 d p_(f), thefraction varying essentially between 1 and the number of detector strips105 a with analyzer gratings 162, i.e. such that

${\frac{p_{f}}{N} < d < p_{f}},$

where N is the number of detector strips 105 a with analyzer gratings162. According to one preferred embodiment, the displacement d isbetween,

${\frac{p_{f}}{3} < d < p_{f}},$

preferably

$\frac{p_{f}}{3}.$

In FIG. 3 a the fringes 163 are represented by the lines in the scandirection y. The darkest sections 163 a of the lines represent thefringe maxima 163 a of the fringe function and the middle of the whitesections 163 b represents the fringe minima 163 b of the function. Thesections surrounding the fringe maxima 163 a thus represent an area ofincreasing/decreasing intensity 163 c of the fringes 163. Hence, thefringe period 163 d is thus defined as the distance between for instancetwo fringe maxima 163 a. The fringes 163 of FIG. 3 a are equal for eachdetector strip 105, which thus schematically shows each detector strip105 a scanning the same point in the object 108 at different points intime. In reality it is namely unlikely that the object 108 would bephase-homogenous over an area in an object 108 corresponding to fourdetector strips 105 a during a scan. Rather, the fringe pattern willvary between each detector strip 105. According to one embodiment, theperiod of the analyzer grating 162 is the same as the period of theinterference fringes 163 P_(f), i.e. wherein the width of the gratingopenings 162 b corresponds to the width of the fringes 163, i.e. halfthe fringe period 163 d, comprising the fringe maxima 163 a and thesection surrounding the fringe maxima 163 a. The pixels 105 p of eachdetector strip 105 a are adapted to sense the intensity of the fringefunction and transmit a corresponding signal to a control unit 121. Inthe figure, such signal may correspond to a sensed fringe maxima 163 aas in pixel 105 p A in detector strip 105 a 1, a sensed intensitycorresponding anywhere between an intensity maxima and minima as forinstance in pixel 105 p A and B in detector strip 105 a 2, or anintensity minima as in for instance in pixel 105 p B of detector strip105 a, as shown in the squares corresponding to the sensed data by thecontrol unit 121. The location of the interference fringes 163 in eachpixel is deduced from the detected signals from a number of detectorstrips at the same point in the object. The displacement of theinterference fringes 163 in each pixel can then be calculated bycomparing to a reference scan. An example of wherein how this can beillustrated is seen in for instance detector strip 105 a and detectorstrip 105 a 2, wherein both signals relating to intensity maxima andintensity minima are detected from pixels 105 p in the same detectorstrips 105 a. Given the constant fringe period 163 d along each detectorpixel 105 p, the only explanation can be that there is a difference inphase shift between the two pixels. The differential phase shift Φ′ inthe object can be calculated according to

${{{\Delta \; p_{f}} \approx {\Lambda \; d_{n} \times \alpha}} = {\Lambda \; d_{n}\frac{\lambda}{2\pi}\Phi^{\prime}}},$

with the fringe displacement Δp_(f) deduced from the displacement of thefringe function over the entire fringe period for a function wherein anobject has been scanned, i.e. placed in the x-ray beam, compared to areference scan, wherein no object or a homogeneous object has beenscanned. The actual phase shift in the object Φ may then obtained byintegration of Φ′, wherein a phase contrast signal may be calculated foreach pixel 105 a.

According to another embodiment, the intensity maxima and minima aresimply arranged in an image corresponding to the object wherein phaseshift occurs, for the operator of the imaging system to interpret andidentify interesting areas by the aid of this information. Theabsorption contrast is detected in the scan direction, i.e. by averagingover the number of detector strips 105 a required to cover an entirefringe period 163 d, wherein the average value of the intensity over thepixels 105 p of these detector strips 105 a generate the absorptioncontrast one position of the image.

FIG. 3 b shows a portion of the detector 105 similar to that of FIG. 3a, and a systematic displacement of the analyzer gratings 162 of twoconsecutive detector strips 105 a with associated analyzer gratings 162,where at least one detector strip 105 a, is arranged between twoconsecutive detector strips 105 a upon which no analyzer grating 162 hasbeen arranged. Hence, the analyzer gratings 162 does not have to bearranged on every detector strip 105 a in the detector 105, i.e. onadjacent detector strips 105 a, and the displacement d can thus bemeasured between two consecutive detector strips 105 a upon which ananalyzer grating 162 is arranged. Further, the order of consecutivedetector strips 105 a with analyzer gratings 162 may be random. However,the total number of detector strips 105 a with analyzer grating 162 mustbe sufficient to cover an entire fringe period 163 d of interferencefringes 163. According to one embodiment, wherein the displacement isset to

$\frac{p_{f}}{3},$

at least three detector strips 105 a would be required.

FIG. 3 c shows a portion similar to that of FIG. 3 a of the detector105, but wherein the displacement d between two consecutive detectorstrips 105 a with analyzer gratings 162 s, in this case adjacentdetector strips 105 a, are displaced relative to each other in thesecond direction in an arbitrary manner, wherein the arbitrary mannercomprises a random displacement. The only restriction on the randomnessof the displacement is that when the summarizing all strips withanalyzer gratings 162 they sample an entire fringe period 163 d ofinterference fringes 163 generated by a phase grating and displaced by aphase gradient in the object 108 when the object 108 is scanned. Thatis, the random displacement needs to be uniformly distributed and rangeover the entire fringe period. In order for phase grating 161 imaging tofunction for a detector 105 with a random displacement of analyzergratings 162, the system needs to be calibrated accordingly such thatthe exact positioning of the analyzer gratings is known by a processingdevice (not shown) which generates the data necessary for displaying animage. Such calibration may for instance be implemented by the placingof a test object with known phase shift in the x-ray beam. According toone embodiment, any type of placement of analyzer gratings, systematicor random, may preferably be calibrated according to this or othermethods. In a similar manner to that of FIG. 3 b, the randomdisplacement of analyzer gratings 162 may occur on a random or arbitraryorder of consecutive detector strips 105 a with analyzer gratings 162 aswell.

FIG. 3 d shows a portion of the detector 105, wherein only two detectorstrips are shown. Analyzer gratings 162 have been arranged in acrosswise manner over the entire length of the detector strips 105 a 2as seen in the figure, such as the grating lines are arranged in thex-direction to cover the entire image field in this direction. Thegrating lines between two detector strips 105 a 2 with crosswisearranged analyzer gratings 162 are displaced a distance d₂ (not shown)over a plurality of detector strips 105 a in the scan direction, suchthat an entire fringe period 163 d in the y-direction is covered by theopenings 162 b of the crosswise analyzer grating 162 ds. Preferably, thedisplacement d₂ is a fraction of the fringe period 163 d P_(f2), in they-direction similar to the displacement d previously described, whereinthe fraction varies essentially between 1 and the number of detectorstrips 105 a 2 with crosswise analyzer gratings 162 d, i.e. such that

$\frac{P_{f}}{N} < d < P_{f}$

where N is the number of detector strips 105 a with analyzer gratings162. According to one preferred embodiment, the displacement d is in therange

${\frac{P_{f\; 2}}{3} < d < P_{f\; 2}},$

preferably

$\frac{P_{f\; 2}}{3}.$

In order to generate the interference fringes 163 in a y-direction for adetector strip with analyzer gratings in this direction, a correspondingsecond phase grating 161 needs to be arranged with grating lines in ay-direction. Hence, the direction of the phase grating(s) need to varywith respect to the direction of the analyzer gratings. According to oneembodiment, such second phase grating 161 may be arranged in theproximity to the phase grating 161 as previously described, in acrosswise manner together with a first phase grating wherein a crosspattern of interference fringes are generated (not shown). Thus, by theaid of this set up, in one single scan, phase contrast detection is nowpossible in two dimensions along with the absorption contrast detection,to further enhance the ability to detect risk areas and abnormalities ina scanned object.

In FIG. 4 a a detector 105 arrangement according to one embodiment ofthe invention is illustrated for enhancing phase contrast information.As has previously been described, the so-called Talbot distances can bedescribed as:

${d_{n} = \frac{D_{n}L}{L - D_{n}}},{where}$${D_{n} = \frac{{np}_{1}^{2}}{2\eta^{2}\lambda}},{n = 1},3,5,$

The photon energy is inversely proportional to the x-ray wave length(λ). The relationship between λ and d_(n) thus implicates that there isan optimal energy E₀ for a given distance between a phase grating 161and the analyzer grating 162. In a fix system according to onealternative embodiment of the invention, wherein the relative distancebetween the phase grating 161 and the analyzer gratings 162 is notadjustable, the efficiency of the phase contrast detection varies withthe energy spectrum of the photons impinging on the detector 105 due tothe varying visibility of the fringe function with the energy. Theenergy spectrum varies for instance with the setting of the accelerationvoltage of the x-ray source 104 by an operator prior to a scan orgradients in breast thickness. Further, variations may also be caused bya so-called automatic exposure control (AEC) when implemented in thesystem, wherein the x-ray flux is optimized based on the breastthickness, sensed by the detector 105 during the beginning of scan byadaptively changing the x-ray source 104 acceleration voltage. An AEC isimportant when acquiring high quality absorption images and musttherefore function parallel to a phase contrast detection functionality.The detector 105 assembly according to FIG. 4 a proposes a detector 105with energy weighting capabilities to overcome this limitation.According to a simplified arrangement, the pixels 105 p of each detectorstrip 105 a in the detector 105 is connected to an amplification block164 a, a comparator block 164 b, and a counter block 164 c. As an x-raysource 104 irradiates an object 108 and the detector 105 with an x-raybeam 122. The x-ray beam 122, containing photons with a certain energyspectrum is filtered by the object, and phase shifts of the photons mayfurther occur. Thus, the photons carry relevant information whenincident onto the detector 105. A signal is created based on the energyof the photon in the detector pixels. The pixel 105 p signals arereadout by first being amplified by an amplifier. After amplificationthe signal may be altered by a band pass filter or shaper, whereinsignal to noise ratio is improved by suppressing high frequencies. Afterbeing amplified, the amplitude of the signal is compared to thresholdlevels in a comparator block, whereupon the comparator outputs 1 if thesignal is above a threshold, and 0 if the signal is below the threshold.A photon pulse counter then increases its number every time the inputchanges from 0 to 1. With the aid of comparators, it is possible tocount each photon having an energy within a certain energy interval.

In FIG. 4 b a shows the energy spectrum of incident photons on adetector according to one embodiment of the x-ray imaging apparatus. Theoptimal energy is denoted E₀ a lower threshold is denoted E₁, a higherthreshold is denoted E₂. Preferably, the photons having an energy withinthe energy interval E₁ to E₂ relevant for phase contrast is assigned ahigher weight for the phase contrast image effect, by either reading outor by using only photons within this energy interval. The setting of theenergy interval in the may be adjustable to comply with non-fix systemswherein the distance d_(n) is adjustable. However, the width of theenergy interval may also depend on the setting of the x-ray source,wherein a higher acceleration leading to a larger flux or intensity ofphotons, adjusts he energy interval to be more narrow and closer to theoptimal energy E₀. Hence, as the flux increases, a higher amount ofphotons with energy close to E₀ will impinge the detector such that thequality of the image may increase. If the flux of photons is low,however, photons within a wider energy interval will have to be includedto enhance the image quality.

According to another embodiment, the content of the photon pulsecounters may be readout to a control unit 121 for optimally weightingthe photons, for image processing and presentation. Preferably, thephotons having an energy within the energy interval relevant for phasecontrast is assigned a higher weight for the phase contrast imageeffect, wherein the photons having an energy within the energy intervalextra relevant for absorption contrast is assigned a higher weight forthe absorption contrast image effect. The weighting is based on pre setcriteria in the control unit, for phase contrast photons may be assigneda 1 if inside the interval, and 0 if outside, and for absorptioncontrast, the photons having energies within the energy interval ofextra importance to absorption contrast, are assigned a value higherthan 0 according to one embodiment. The setting of the energy intervalsmay be adjustable to comply with non-fix systems wherein the distanced_(n) is adjustable. This requires a set up of comparators and countersadapted to count photons within two energy intervals, a first energyinterval defined by first lower threshold and a second higher threshold,and a second interval defined by a first lower threshold and a second,higher threshold. Assigning a higher weight, may, especially for thecase of phase contrast photons comprise weighting by the factor 1.

According to another embodiment, three energy levels are used to countand weight photons according to increase phase contrast and absorptioncontrast effects, essentially dividing the energy spectra into threeenergy intervals, wherein the upper and lower limits are defined byinfinitely high energies and 0 respectively. Preferably, photon withenergies within the lower energy interval is filtered out and used forphase contrast. Photons with energies within the middle energy intervalis counted and/or weighted positive, i.e. assigned a higher weight forabsorption contrast and photons within the upper energy interval iscounted and assigned a higher weight for phase contrast. The weightingof phase contrast photons may include weighting with a factor 1.

FIG. 5 shows a scan arm 103 with an x-ray source 104 and a detector 105arranged at two positions, 107 a and 107 a 2 respectively, essentiallyin each end of the scan arm 103. As known from previous figures, e.g.FIG. 2 a, a phase grating 161 and pre-collimator 106 a is arrangedbetween the x-ray source 104 and the detector 105. Further, between thex-ray source 104 and the detector 105 a compression paddle 107 a, 107 bis arranged to move and/or compress the object 108 such as a breast in avertical direction. In imaging arrangements to the present date, acompression paddle 107 a, 107 b is used for pressing the breast towardsdownwards towards a second compression paddle 107 a, 107 b, also knownas the object 108 table. However, in order to increase the effect of thephase contrast in the x-ray image, the breast needs to be placed as faraway from the detector 105. As described, For small α, the refractioncauses a fringe displacement

${{\Delta \; p_{f}} \approx {\Lambda \; d_{n} \times \alpha}} = {\Lambda \; d_{n}\frac{\lambda}{2\pi}\Phi^{\prime}}$

at a distance Λd_(n) from the object, where Λ ranges from 0 for anobject 108 placed in contact with the detector 105 to 1 for an object108 at or upstream of the phase grating 161. Thus, the farther away fromthe detector 105, the larger the fringe displacement detectable by thedetector 105. There is a trade-off between the absorption effect and thephase contrast effect depending on the vertical distance of the scannedobject 108 from the analyzer grating 162. An increased distance from thedetector 105 will diminish the absorption contrast effect due toscattering effects wherein valuable radiation that has passed the object108 is lost. Therefore, the height of the compression paddles 107 a, 107b should be adjusted based on and prior to the preferred type of scan tobe performed. This could be implemented such that the height isautomatically adjusted based on the setting by an operator of the x-rayimaging system 101.

In the field of mammography, there is an increasing demand forthree-dimensional (3D) information which can reduce distraction byanatomical structures and provide 3D localization. The proposedembodiments disclosed in this application could readily be implementedin known tomosynthesis solutions, wherein projection angles aregenerated with the purpose to create projection images when the x-raysource 104 irradiates each point in an object 108 from various angles.

The present invention should not be limited to extracting phase-contrastinformation from the detected signals of the interference fringes. Oneexample of other information that may be available is information aboutthe object scattering ability, so-called dark-field imaging, such as in.In dark-field imaging, the visibility of the detected periodic function,defined as

$V = \frac{I_{\max} - I_{\min}}{I_{\max} + I_{\min}}$

where I_(max) and I_(min) are the intensity maxima and minimarespectively, may be compared to the visibility of a reference scan andused to obtain the dark-field image.

1. An x-ray imaging system comprising: an x-ray source; an x-raydetector comprising a plurality of detector strips arranged in a firstdirection of the x-ray detector, each detector strip further comprisinga plurality of detector pixels arranged in a second direction of thex-ray detector; a phase grating; and a plurality of analyzer gratingscomprising grating slits; wherein the x-ray source and the x-raydetector are adapted to perform a scanning movement in relation to anobject in the first direction, in order to scan the object; wherein theanalyzer gratings are arranged between the x-ray source and the x-raydetector, wherein each of the plurality of analyzer gratings is arrangedin association with a respective detector strip with the grating slitsarranged in the second direction and wherein the grating slits of theanalyzer gratings of the detector strips are displaced relative to eachother in the second direction.
 2. An x-ray imaging system according toclaim 1, wherein the displacement of grating slits of the analyzergratings along a plurality of detector strips samples an entire fringeperiod of interference fringes generated by a phase grating anddisplaced by a phase gradient in the object when the object is scanned.3. An x-ray imaging system according to claim 1, wherein the gratingslits of analyzer gratings of two consecutive detector strips withanalyzer gratings in a first direction are displaced relative to eachother in the second direction in a systematic manner, wherein thesystematic manner comprises a defined displacement distance.
 4. An x-rayimaging system according to claim 3, wherein the displacement distanceis a fraction of the fringe period p_(f), such that${\frac{p_{f}}{N} < d < p_{f}},$ where N is the number of detectorstrips such that the entire fringe period is covered.
 5. An x-rayimaging system according to claim 3, wherein the displacement distanceis between, ${\frac{p_{f}}{3} < d < p_{f}},$ preferably$\frac{p_{f}}{3}.$
 6. An x-ray imaging system according to claim 1,wherein the grating slits of analyzer gratings of two consecutivedetector strips are displaced relative to each other in the seconddirection in an arbitrary manner, wherein the arbitrary manner comprisesa random displacement.
 7. (canceled)
 8. An x-ray imaging systemaccording to claim 1, wherein two consecutive detector strips withanalyzer gratings in a first direction are two adjacent detector strips.9. An x-ray imaging system according to claim 1, wherein two consecutivedetector strips with analyzer gratings in a first direction are randomlyor arbitrarily displaced among the detector strips in a first direction.10-11. (canceled)
 12. An x-ray imaging system according to claim 1,wherein the system further comprises a pre-collimator and apost-collimator, wherein the pre-collimator is arranged between theanalyzer grating and the phase grating and the post-collimator isarranged between the analyzer grating and the x-ray detector.
 13. Anx-ray imaging system according to claim 1, wherein the system furthercomprises a source grating arranged between the x-ray source and thephase grating.
 14. An x-ray imaging system according to claim 1, whereinthat the detector is adapted to count photons impinging on the detectorstrips and generate a signal corresponding to the energy of impingingphotons, and wherein a control unit is adapted to receive said signalsand assign a weight to the phase-contrast image effect in relation tothe efficiency at each energy and/or wherein the control unit is adaptedto assign a weight to the phase-contrast image effect in relation to theefficiency at each energy. 15-19. (canceled)
 20. An x-ray imaging systemaccording to claim 1, wherein at least one analyzer grating is arrangedin a first crosswise direction over the entire detector.
 21. An x-rayimaging system according to claim 1, wherein the system furthercomprises at least one movable compression paddle, wherein thecompression paddle is adapted to move an object, such as a breast,further away from the analyzer grating to increase the phase contrastimage effect.
 22. An x-ray imaging system according to claim 21, whereinthe compression paddle is adapted to move the object within a rangebetween the analyzer grating and the pre-collimator or the phasegrating. 23-25. (canceled)
 26. An x-ray imaging system according toclaim 1, wherein the phase grating is arranged on the scan arm in orderto follow the scan arm during the scanning movement in relation to theobject in the first direction, and/or the phase grating (161) isarranged to be stationary when the scan arm (103), during the scanningmovement, is moved in relation to the object (108) and the phase grating(161) in the first direction. 27-28. (canceled)
 29. An x-ray imagingsystem according to claim 1, wherein the analyzer grating is connectedto each of the plurality of detector strips (105 a) by a snap-fit likeconnection.
 30. An x-ray imaging system according to claim 1, whereinthe analyzer gratings are arranged directed towards the x-ray source,wherein the tilting direction of the analyzer gratings are essentiallyequal to the tilting angle(s) of the plurality of detector strips inrelation to the x-ray source.
 31. An x-ray imaging system according toclaim 1, wherein each analyzer grating comprises several smaller units,adapted to be connected to each other during the manufacturing of theanalyzer gratings.